1. Field of the Invention
This invention pertains to the acquisition of seismic imaging reflection data for input to a 3-D migration data process particularly suited for subsurface stratigraphic structures having relatively steep dip with approximate circular symmetry.
2. Description of the Prior Art
Diapirs and domes, which dip steeply in all directions from an approximate central position or peak, are structures commonly of interest in seismic prospecting. Experience has taught that 2-D common midpoint reflection data of optimum data quality can be achieved by orienting profiles radially, i.e., in the direction of maximum dip. The surface spatial sampling required to properly image the earth's subsurface through the process of seismic migration is governed by the severity of dip and the velocity of propagation through the earth. By experience, it has been learned that wide apertures and tight sampling intervals are necessary to resolve steeply dipping seismic targets. The acquisition and processing of such information is economically efficient to obtain for 2-D surveys.
Although the line orientation described above improves 2-D data quality for steeply dipping events, these structures are commonly accompanied by radial faulting that corrupts the data. Such faults are poorly resolved by such technique. Hence, further improvements in data quality are typically pursued by conducting a 3-D survey. With conventional 3-D survey methodology, common midpoint data is acquired in parallel lines throughout the area of interest. A rectilinear sampling, or so-called "binning", scheme is used to sort and stack the data for input to a 3-D migration process. Small, usually square "bins" are required to provide adequate sampling since nothing is incorporated in the procedure allowing for a dominant dip direction. This is so, even though through prior or preliminary data gathering it is known that there are dominant dips in multiple directions from a central position.
Two primary disadvantages are evident with the parallel method just described. First, the majority of the common midpoint reflection data that is collected contains the reflected energy from the targets of interest on the flanks of the structure as sideswipe. Such acquired data quality is degraded when compared with data acquired in the direction of dominant dip. Further, velocity analysis is complicated for such acquired data. The second primary disadvantage to the parallel method is the very high cost involved as a result of the dense surface coverage required to result in valid data for migration processing. In order to satisfy proper sampling requirements at all times, much of the surface area must be oversampled because sampling is not related to the structure of interest to be imaged.
Therefore, it is a feature of the present invention to provide an improved method of acquiring seismic data related to a seismic subsurface known to be approximately circular in character by acquiring data in radial patterns from the center of such subsurface, the data acquired being efficiently sampled for the steep dip structure with respect to the center and not resulting in oversampled data not related to the dip structure.
It is another feature of the present invention to provide an improved method of acquiring seismic data for migration processing by acquiring polar symmetrical data with respect to a center position of an area that dips in all directions therefrom, the data being acquired and sorted in annular sectors of approximately the same size regardless of how far an individual sector is away from the center.